Add Price and Extended price to Project Bom

[whinis]

This seems to be one of the most requested changes and I also needed this particular feature.
requested in #603 and others in discussion. So this allows one to see the price of each item in the bom if it exists in any way. It currently selects just the first supplier and the first (likely most expensive) unit price for the part.

[jbtronics]

Thanks,

The PricedetailHelper has an calculateAvg() price to calculate an average price across all suppliers, it also handles currency conversions correctly.
Also you should use the correct Intl Number Formatter to get the correctly formatted price for users locale. The currency symbol should be included, to mark which currency this is.

Unfortunatley the whole price infrastructure in Part-DB is quite limited and ideally would need some heavy rewrites, with some more powerful functions to make things like this easier...